From what I see, practicing Dark Arts is not really a crime except for the Unforgivable Curses.

Although many members of the wizarding world appear to frown upon the Dark Arts, they are not prohibited (with the exception of the Unforgivables) and may even — under certain conditions — be encouraged. For example, while Hogwarts School of Witchcraft and Wizardry teaches only Defence Against the Dark Arts, Durmstrang Institute teaches the Dark Arts itself. Moreover, shops like Borgin and Burkes in Knockturn Alley openly trade in Dark artefacts, and are patronised by "hundreds of people",[5] which suggests that at least some aspects of the Dark Arts are somewhat socially acceptable or at the very least legal among parts of the wizarding population.

Also, in times of war, the Ministry of Magic has been known to permit Aurors to use strong Dark magic against its enemies. When Lord Voldemort was in control of wizarding Britain in late 1997 to mid-1998, every aspect of the Dark Arts was legalised and taught at Hogwarts by Death Eater professor Amycus Carrow. However, Voldemort was controlling the Ministry at this time, so it may not be standard protocol.[3]

In addition, there is a matter of degree. The three Unforgivable Curses are punishable by an automatic life sentence in Azkaban Prison[6] when used on humans, except when permitted in wartime, whereas some types of magic can have more benign uses as well as Dark uses and, thus, are presumably not always illegal.

Well, if so, if Voldemort never kills (including the creation of Horcruxes), and he, instead, legally remotely or personally controlled the Ministry in order to achieve his goals, his and his Death Eaters' actions are technically legal. Right? No?

yeah aside from those.. you know like politically, using death eaters power in the ministry and other legal dark arts, achieve his goals
– LordSachaNov 27 '16 at 5:01

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How could Voldemort legally control the Ministry without the use of the Dark Arts or other morally questionable activities? Sure, he has some Death Eaters who are well-connected, but not nearly enough to outright control the government. Your title asks what, besides the Unforgivable curses, gets Voldemort’s allies locked up in Azkaban, but your question body seems to posit a situation in which Voldemort somehow pursues his campaign of terror and pureblood supremacy without violence.
– AdamantNov 27 '16 at 5:05

3 Answers
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We know that murder is illegal because Sirius Black was sent to Azkaban for the murder of Peter Pettigrew. We also know that this wasn't because he uses Avada Kadavera because Pettigrew was apparently blown to pieces and AK doesn't cause any physical damage.

Another example is Hagrid getting sent to Azkaban just on fairly flimsy suspicion of opening the Chamber of Secrets even before it is very clear exactly what is going on. There are also lots of other examples which suggest that the magical justice system is quite politicised. We know that a significant number of Death Eaters were imprisoned after the first war and the ministry authorised the use of the dementor's kiss on Black in PoA which effectively amounts to summary execution.

Saying if Voldemort never killed anyone is a big IF as we are directly shown him killing and torturing several people as well as more he hear of indirectly and yet more killed on his orders. Equally threats and coercion seem to be his preferred strategy. It's not even that clear that Voldemort is really after political power as his ultimate goal, most of his specific motivations seem to be about protecting himself and eliminating potential opposition. The sense I get is that he his more interested in simply being able to do what he personally wants rather than acquiring power on other people's terms. Indeed it's not at all far fetched that if he just wanted to by Minister of Magic he have more than enough talent and motivation to do so on his own merits without ever breaking the law.

We are never really told much about the details of how the legal and political systems work so I suppose that he could have infiltrated the ministry using more conventional political means but that is certainly not what happened.

We also know that certain other spells like Morsmordre are illegal from the start of GoF.

Bear in mind also that he attacked a school, half demolishing it in the process and killing dozens of people, including children. It would be a very strange legal system which would let him off that because he unilaterally had the law changed after a coup.

There is never really a formal definition of 'dark arts' in the books but we are told that Malfoy Manor has been searched for dark objects on several occasions so presumably there are prohibited objects. This is complicated a bit by the fact that there is quite a lot of tolerance for dangerous objects so the line between dangerous and dark is not entirely clear. Similarly with Borgin and Burke, when we see the Malfoys visit the shop they are very furtive and apparently don't want to be seen there which suggests that its association with dark objects is not completely socially acceptable and the overall impression is that the shop is just barely on the right side of the law.

Similarly Arthur Weasley's job involves policing magically modified muggle artefacts. These mostly seem to be fairly trivial items (a regurgitating toilet is mentioned) but they are certainly illegal and there is talk of fines and imprisonment.

A much more in-depth answer than my own :)
– DBPriGuyNov 28 '16 at 5:01

"he unilaterally had the law changed after a coup." Well ... strictly speaking, Harry Potter led a coup. Voldemort led a force to retake Hogwarts from those who had illegally taken it over. "Similarly Arthur Weasley's job involves policing magically modified muggle artefacts." That's just an extension of the Statute of Magical Secrecy.
– AcccumulationApr 19 '18 at 21:26

I'm gonna go out on a limb here and say yes, there are reasons for Voldemort and his Death Eaters to be sent to Azkaban outside the use of Unforgivable Curses. I'm only going to provide two examples because frankly, there are too many to easily count. I will leave it to a more ambitious SF&F user to provide a comprehensive lists of non-Unforgivable crimes committed by Lord Voldemort and his Death Eaters.

First - murder (without the use of the Killing Curse): Voldemort murdered Severus Snape using his snake, Nagini.

Second - kidnapping: Ollivander the Wandmaker was kidnapped by Voldemort and his Death Eaters, held captive, and tortured to force him to divulge his knowledge of wandlore.

I will admit that I do not have canon evidence that these two actions are illegal, but I think it is reasonable to assume that these two actions, especially murder, are illegal in just about any legal system.

They committed crimes other than using Unforgivable Curses.

Over the course of their attempt to take over the wizarding world, the Dark Lord and the Death Eaters did commit other crimes known to carry sentences in Azkaban that were unrelated to the Unforgivable Curses or the Dark Arts. Some of the Death Eaters were actually convicted of things other than using Unforgivable Curses.

Breaking into the Ministry and attempted theft:

Sturgis Podmore was convicted and sentenced to six months in Azkaban for trespass and attempted robbery after breaking into the Ministry.

“Sturgis Podmore, 38, of number two, Laburnum Gardens, Clapham, has appeared in front of the Wizengamot charged with trespass and attempted robbery at the Ministry of Magic on 31st August. Podmore was arrested by Ministry of Magic watchwizard Eric Munch, who found him attempting to force his way through a top-security door at one o’clock in the morning. Podmore, who refused to speak in his own defence, was convicted on both charges and sentenced to six months in Azkaban.” - Harry Potter and the Order of the Phoenix, Chapter 14 (Percy and Padfoot)

Some of the Death Eaters broke into the Ministry and attempted to steal the prophecy - a very similar situation to what Podmore was convicted for.

Setting dangerous creatures on people:

Hagrid was thrown in Azkaban for allegedly setting a creature on students. No one died from it, but setting the creature on students was enough to be considered a crime and worth a sentence in Azkaban.

“They think, up at the school, that Hagrid’s been setting a – a – something on students. They’ve taken him to Azkaban.” - Harry Potter and the Chamber of Secrets, Chapter 15 (Aragog)

The Dark Lord had set Nagini on Arthur Weasley, and as a student at Hogwarts, the basilisk on students, resulting in the death of Moaning Myrtle.

Attacking Muggles:

Attacking Muggles seems to be a crime that carries a sentence in Azkaban, however it’s done. Morfin Gaunt was convicted for making Tom Riddle break out in hives - clearly not one of the Unforgivable Curses.

“Ogden Apparated back to the Ministry and returned with reinforcements within fifteen minutes. Morfin and his father attempted to fight, but both were overpowered, removed from the cottage and subsequently convicted by the Wizengamot. Morfin, who already had a record of Muggle attacks, was sentenced to three years in Azkaban. Marvolo, who had injured several Ministry employees in addition to Ogden, received six months.” - Harry Potter and the Half-Blood Prince, Chapter 10 (The House of Gaunt)

Attacking Muggles seems to get two different charges - violating the Statute of Secrecy by doing magic in front of Muggles, as well as causing harm to the Muggles.

“He glanced down at his scroll of parchment again. ‘Morfin will attend a hearing on the fourteenth of September to answer the charges of using magic in front of a Muggle and causing harm and distress to that same Mugg—’” - Harry Potter and the Half-Blood Prince, Chapter 10 (The House of Gaunt)*

A group of Death Eaters attacked some Muggles during the Quidditch World Cup without using any Dark magic - they levitated them.

Leaking Ministry of Magic secrets:

One Death Eater, Augustus Rookwood, was convicted of telling the Dark Lord the Ministry’s secrets.

“Augustus Rookwood, said the caption beneath a pockmarked man with greasy hair who was leaning against the edge of his picture, looking bored, convicted of leaking Ministry of Magic secrets to He Who Must Not Be Named.” - Harry Potter and the Order of the Phoenix, Chapter 25 (The Beetle at Bay)

This didn’t involve any Unforgivable Curses or violent crimes, but it got Rookwood sentenced to Azkaban for what seemed to be several years. This would likely still be a crime even if neither the Dark Lord nor the Death Eaters used Unforgivable Curses - it’s probably a crime to leak Ministry information to anyone who’s not supposed to know it.

Being an unregistered Animagus (Wormtail only):

Despite three of the Marauders doing it, it’s actually a crime to be an unregistered Animagus.

“Otherwise, as you very well know, I will inform the authorities that you are an unregistered Animagus. Of course, the Prophet might give you rather a lot for an insider’s account of life in Azkaban.” - Harry Potter and the Order of the Phoenix, Chapter 25 (The Beetle at Bay)

Only Wormtail was an unregistered Animagus, but he could be sent to Azkaban for it.

They perhaps could legally rule the Ministry by changing strategy.

The Dark Lord could possibly legally rule the Ministry - the Ministry only has to answer to the International Confederation of Wizards, and other than that, the Minister seems basically able to do what he wants apart from breaking the Statute of Secrecy. However, he’d need other Ministry wizards who were willing to go along with him, otherwise he’d just be removed as Minister. It’d likely be more difficult to get enough of his supporters legally among the ranks of the Ministry - to legally take it over he can’t use the Imperius Curse, so the Death Eaters would need to actually become Ministry employees. The Dark Lord would need to legally get enough of his supporters into the different departments before he’d be able to start making drastic policy changes, so he could remain in power.

"Attacking Muggles seems to be a crime that carries a sentence in Azkaban, however it’s done.“Scarcely a year previously, his father, Percival, had been convicted of a savage and well-publicised attack upon three young Muggles."" If it was well-publicized among muggles, then it might have been more the Statute of Secrecy issues than the assault.
– AcccumulationApr 19 '18 at 21:30

@Acccumulation That makes sense - however, I think he meant among wizards. That quote is from Elphias Doge, when he was talking about how Dumbledore coped with his father’s notoriety. There were also the cases where Morfin and Marvolo attacked Muggles - Morfin made Tom Riddle break out in hives. They were charged with both doing magic in front of Muggles as well as causing harm to the Muggles.
– BellatrixApr 19 '18 at 21:45